Biden has an EO that makes it mandatory to wear masks on Federal property. The military is also NOT required to get the vaccine (yet). Also, you have to realize that officially Biden is POTUS when it comes to the military. There really isn't some hidden knowledge going around the military that Trump is still President. For all intents and purposes (especially to the rank & file), Biden is in their Chain of Command.

Biden has an EO that makes it mandatory to wear masks on Federal property. The military is also NOT required to get the vaccine (yet).